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SOME EARLY APPLICATIONS OF PHOTOGRAPHY 215
employed a similar instrument to determine the temperature in the depths of the sea. As there is no light producing chemical action at those depths, Neumeyer sent down a light-producing apparatus. This consisted of a galvanic batter}^ and a Giesler's tube — that is, a tube in which very attenuated nitrogen gas is enclosed, and through which the electric current is passed. The tube then gives out a faint light. But this faint light has a powerful chemical action, because it contains many of the invisible ultra-violet rays (see p. 168), and in three minutes it effects the blackening of the paper. Neumeyer also attempted to determine with his apparatus the direction of the oceanic currents. For this purpose the apparatus had attached a vane like that of a weathercock, which could move the instrument in any direction whilst suspended. If any currents exist, they will turn the apparatus so that the vane is parallel to their direction. A magnetic needle was enclosed in the apparatus, and moved over a disc of sensitive paper ; this magnetic needle pointed, of course, to the magnetic north, and the luminous tube above it fixed its position on the sensitive paper, which was firmly fastened to the box. Therefore, it can be easily seen what situation the apparatus has assumed with reference to the magnetic needle.
Stein s Heliopictor and other Apparatus. — Medical science soon made great use of photography, both as a means of obtaining pictures of interesting anatomical preparations and phenomena of short duration, and in giving exact anatomical views of the different organs. The interior of living organs can be disclosed by ophthalmoscopes, otoscopes, and laryngoscopes, and the image seen by the eye with these instruments successfully photographed. Dr Stein, of Frankfort-on-the-Maine, did good service in this branch, not only as a practical photographer, but also by the construction of suitable apparatus.
The apparatus devised by him for photographing the